This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Alpha-beta T cell receptors (TCRs) recognize peptide antigens bound and presented by major histocompatibility complex (MHC) proteins. Although T cell receptor cross-reactivity is a fundamental property of the immune system and is implicated in the immune response to cancer and numerous autoimmune pathologies, the molecular mechanisms by which TCRs can recognize and respond to diverse ligands are poorly understood. In our NIH and ACS funded work, we are seeking insight into TCR cross-reactivity through investigations of the structural and biophysical properties of TCR-pMHC interactions, and our studies involve structural determination of TCR-peptide/MHC complexes as well as unligated TCRs and peptide/MHC molecules. Structural properties are related to biophysical and immunological data. Recent findings we aim to build on with additional synchrotron time include: 1) observations that TCR recognition can proceed with cooperative structural changes occurring on both sides of the interface (Gagnon et al., J Mol Biol 363 2006 and unpublished crystallographic data) and 2) observations that subtle substitutions in antigenic peptides can have complex structural consequences, the immunological consequences of which are difficult to reconcile with current models of TCR recognition (Borbulevych et al., J Mol Biol 372 2007). The latter observations are of particular interest in the design of altered peptides for use in cancer immunotherapy (e.g., Borbulevych et al. J Immunol 174 2005). Experiments planned for the near future include structural studies of different TCRs bound to the same peptide/MHC complex and studies of peptide/MHC variants designed to elicit improved immunological responses with T cells specific for the tumor antigen Melan-A/MART-1.